Mucin synthesis. III. UDP-GlcNAc:Galβ1-3(GlcNAcβ1-6)GalNAc-R (GlcNAc to Gal) β3-N-acetylglucosaminyltransferase, an enzyme in porcine gastric mucosa involved in the elongation of mucin-type oligosaccharides

Abstract

Pig gastric mucosa micosomes catalyze the following reaction: UDP-GlcNAc + Gal.beta.1-3(GlcNAc.beta.1-6)-GalNAc-.alpha.-R .fwdarw. GlcNAc.beta.1-3Gal.beta.1-3(GlcNAc.beta.1-6)GalNAc-.alpha.-R + UDP, where R is o-nitrophenyl or benzyl. The enzyme catalyzing this reaction has been named UDP-GlcNAc:Gal.beta.1-3(GlcNAc.beta.1-6)GalNAc-R(GlcNAc to Gal) .beta.3-N-acetylglucosaminyltransferase. The .beta.3-GlcNAc-transferase does not act on Gal.beta.1-3GalNAc-.alpha.-o-nitrophenyl. The .beta.3-GlcNAc-transferase requires Mn2+ and Triton X-100 for optimal activity. The Vmax for the microsomal enzyme is 8.7 nmol/mg protein per h and the Km values are 1.6, 0.9, and 1.2 mM for UDP-GlcNAc and the .alpha.-o-nitrophenyl and .alpha.-benzyl derivatives of Gal.beta.1-3(GlcNAc.beta.1-6)GalNAc, respectively. Pig gastric mucosa microsomes catalyze the transfer of GlcNAc to lactose to form GlcNAc.beta.1-3Gal.beta.1-4Glc, but fail to transfer GlcNAc to lactosyl ceramide, Gal.beta.1-4GlcNAc, or Gal.beta.1-4GlcNAc-.beta.-benzyl.